Transparent resins with good mechanical characteristics have been used frequently as optical materials for engineering plastics. For example, polymethyl methacrylate (hereinafter referred to as PMMA) and polycarbonate (hereinafter referred to as PC) are used as optical materials for compact discs, laser discs, lenses, etc., and also as transparent parts of automobiles. PMMA has the advantage of good transparency and small optical anisotropy, but is disadvantageous in that it is highly hygroscopic. Therefore, moldings of PMMA are often deformed and have poor shape stability. On the other hand, PC has the advantage of good transparency and good heat resistance, but is disadvantageous in that its fluidity is low and therefore, the birefringence of its moldings is often large. For those reasons, PMMA and PC are not satisfactory for use as optical materials. The recent development of optical discs for recording and reproducing various information such as sounds, images and letters by the use of laser rays is advancing such that substrate materials having optical characteristics of higher quality are desired.
Using polyester copolymers as optical materials was proposed (see Japanese Patent Publication (JP-B) Sho-57-20864, Sho-2-98845 and Hei-2-38428), but the resins proposed are not always satisfactory as their optical characteristics are often not good.
In JP-B Hei-3-168211, various polyester resins comprising 9,9-bis(4-hydroxyphenyl)fluorene, 2,2-bis(4-hydroxyphenyl)propane, terephthalic acid and isophthalic acid were described as having a glass transition point which is sufficiently high and that an optical anisotropy of oriented films of such resins is small.
In U.S. Pat. No. 3,546,165, the production of a polymer of 9,9-bis(4-hydroxyphenyl)fluorene, terephthalic acid and isophthalic acid in an acid chloride method is described. As an improvement to the polymer, a heat-resistant polyester comprising 9,9-bis(4-hydroxyphenyl)fluorene, terephthalic acid, isophthalic acid and a fatty acid in a specific ratio is disclosed.
However, all of the known polymers described above comprise 9,9-bis(4-hydroxyphenyl)fluorene, which requires special conditions for polymerization thereof. Specifically, since 9,9-bis(4-hydroxyphenyl)fluorene has a phenolic group at both terminals, being different from aliphatic alcohols, it is extremely difficult for it to react. Therefore, it requires reaction conditions at higher temperatures in its melt polymerization, in which, however, the polymer formed is easily pyrolyzed, colored and deteriorated. In addition, when the monomer is polymerized with a dicarboxylic acid in a method where the dicarboxylic acid is previously converted into an acid chloride thereof at its terminal groups and is thereafter polymerized with the monomer in a solvent while removing hydrochloric acid formed, it is difficult to obtain a uniform reaction therebetween, resulting in the polymer formed having a wide molecular weight distribution. Moreover, the polymerization of the monomer in question requires a large amount of a catalyst compound, and the post-treatment of the catalyst used is troublesome. Thus, the method of polymerizing the monomer is complicated and is expensive. Further, despite various improvements to the polymerization of the monomer, the polymer formed has too high of a glass transition point and its moldability is still unsatisfactory. As a result, for example, injection molding of the polymer is difficult.